1. Technical Field
This invention relates generally to exhaust gas recirculation systems and methods of operation and more particularly to such systems and methods applicable to Diesel engines.
2. Background Art
Diesel engines play an important role in power generation and transportation economies. However, NOx and soot emissions from both stationary and mobile Diesel engines are a major contributor to air pollution. In order to meet future NOx emissions requirements, heavy-duty Diesel engines are looking to utilize exhaust gas recirculation (EGR) technology as a means of lowering NOx emissions. This technique directs some of the exhaust gases back into the intake of the engine. These gases have already been used by the engine and no longer contain a great amount of excess oxygen. By mixing the exhaust gases with fresh air, the amount of oxygen entering the engine is reduced. Since there is less oxygen to react with, few nitrogen oxides are formed. This greatly reduces the amount of nitrogen oxides that a vehicle releases into the atmosphere. Oxides of nitrogen are formed in the combustion chamber when the combustion temperature exceeds at 2500° F., or hotter, and nitrogen and oxygen in the combustion chamber more readily combine to form nitrous oxides. Controlled exhaust gas recirculation is able to lower the combustion temperature, often below the point at which nitrogen combines with oxygen to form NOx. However, if the combustion temperature is too low, combustion becomes unstable, resulting in the formation of soot, i.e., particulate matter (PM), an undesirable emission.
Therefore, combustion temperature control of the exhaust gas recirculation equipped Diesel engines, although not fully addressed in prior EGR systems, is extremely important. Heretofore, EGR systems have primarily been directed to exhaust gas regulation rate control. For example, U.S. Pat. No. 6,305,167 granted Oct. 23, 2001 to Weisman, II et al. for a METHOD OF CONTROLLING AN ENGINE WITH AN EGR SYSTEM varies the exhaust gas recirculation rate by controlling the exhaust gas flow rate through a variable geometry turbocharger. However, the Weisman, et al. system cannot control combustion temperatures independently of exhaust gas recirculation flow rate.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an exhaust gas recirculation system, and a method for operating the system, that enables the control of combustion temperatures under high load operation to reduce NOx formation, and maintain a sufficiently high combustion temperature during light load operation to prevent combustion instability and soot formation. Also, it is desirable to have such a system and method of operation that maintains the bed temperature of a catalytic exhaust gas aftertreatment device at a temperature sufficient to promote the efficient conversion of undesirable exhaust products.